add iteration-count to `eval::IO` (someday)

[porcuquine]

Someday, Nova will not require num_steps for verification. When that happens, we will want to move iteration-count into the frame I/O and explicitly allow for hiding or revealing the count according to application needs. Do that then.

#278 notes the following:

// Currently, claims created by fcomm don't include the iteration count. If they do, then it should be
// possible to verify correctness. This may require making the iteration count explicit in the public
// output. That will allow maintaining iteration count without incrementing during frames added as
// padding; and it will also allow explicitly masking the count when desired for zero-knowledge.
// Meanwhile, since Nova currently requires the number of steps to be provided by the verifier, we have
// to provide it. For now, we should at least be able to calculate this value based on number of real
// iterations and number of frames per circuit. This is untested and mostly a placeholder to remind us
 // that all of this will need to be handled in a more principled way eventually. (#282)